In oil exploration and geophysical survey, it is useful to provide measuring systems which can detect locations of boundaries between different formations. Examples of applications of such systems include reservoir characterization, geo-steering in horizontal drilling, salt dome mapping for engineering of gas storage caverns and delineation of hydrocarbon traps, waste disposal control, and rock fracture detection in environmental logging. There are several advantages of using transient electromagnetic (radar) signals in such measurements. First, the arrival time of reflected signals is a direct measure of boundary locations, if the wave speed of the background medium is known. Second, the pulse can have a narrow temporal width which gives high spatial resolution.
Generally, radar measurements comprise transmitting an electromagnetic signal from a transmitter which is reflected from a target structure of interest and the reflected signal is detected at a receiver. In general terms, for a radar system to be able to determine the distance to the structure, the space between the transmitter and receiver must be known together with the speed of the signal through the medium separating the transmitter/receiver and the target structure. In cases where the signal passes through air, the wave speed is known and does not vary significantly with conditions. However, measurement of the wave speed in underground formations has been difficult and can vary significantly. Borehole logging tools utilizing the principle of radar have been proposed in U.S. Pat. No. 4,670,717, U.S. Pat. No. 4,814,768, U.S. Pat. No. 4,297,699, U.S. Pat. No. 4,430,653 and GB 2,030,414. Some of the previously proposed techniques assume values for the wave speed to allow interpretation of the radar signal. For example, in U.S. Pat. No. 4,814,768 the wave speed is assumed to be the speed of an electromagnetic wave in water. Other techniques rely on separate measurements to provide the wave speed. These approaches suffer from certain problems. Where a value is assumed, the actual speed can vary significantly from formation to formation relative to this value making interpretation of the signals inaccurate. Where the speed is obtained from other measurements, the operation takes longer and is more expensive.
It is an object of the present invention to provide a method and apparatus which overcomes or avoids the above identified problems. The present invention allows the formation wave speed to be obtained from the measurements made by a borehole radar tool.
The present invention achieves this object by measuring the arrival of a direct signal from a transmitter to a receiver to obtain the formation wave speed. It is a further object of the invention to provide a tool in which the time of arrival of the direct signal can be measured.